Electrical Impedance Tomography (EIT) is an imaging modality that estimates the electrical impedance in the interior of an object from voltage measurements made on its surface by injecting currents via surface electrodes (Barber and Seagar, 1987; Metheral et al., 1996). However, the accuracy and spatial resolution of EIT are currently limited because the boundary voltage measurements are not sensitive to the change in conductivity within the body.
In magnetoacoustic tomography (MAT) (Towe & Islam, 1988; Islam & Towe, 1988; Roth et al., 1994) and the reverse mode—Hall effect imaging (HEI) (Wen et al., 1998), the sample is located in a static magnetic field and current is injected into a sample by applying electrodes on the surface of the sample, and acoustic signals are collected around the object. However, there has been no report on quantitative image reconstruction of electrical impedance.
Magnetic Resonance Electrical Impedance Tomography (MREIT) integrates Current Density Imaging (CDI) (Joy et al., 1989; Scott et al., 1991) and EIT (Barber and Seagar, 1987; Metheral et al., 1996). In MREIT (Woo et al., 1994; Kwon et al., 2002; Gao et al., 2005), a low frequency current is injected into a body through pairs of surface electrodes, and the distribution of the induced magnetic flux density inside the body is measured by a magnetic resonance imaging (MRI) system. The current density distribution inside the body can be obtained using Ampere's law. The body's conductivity distribution can be calculated from the relationship between conductivity and current density. MREIT has been pursued by injecting electrical currents using surface electrodes, with subject being placed within the MRI scanner. The magnetic flux density is measured which is then used to construct current density or impedance distribution within the body. Disadvantages of this method include contact with a patient, pain from current injection, errors caused by incorrectly positioning electrodes, and difficulty injecting currents into the brain due to the low conductivity of the human skull.